The very young MG deficient mammal may experience a shock-like syndrome with apnea, bradycardia, cyanosis, respiratory distress, and with a sharp decrease in blood pH and some normalization of the plasma Mg level. Premature neonates with similar episodes may be human counterparts. At the NICHD, we are analyzing data from an 8-year study in St. Louis of 251 infants with unexplained apnea, usually with bradycardia. Parenteral Mg load testing and/or plasma values were usually diagnostic of Mg deficiency. Premature infants who received 0-4 days of Mg therapy (Group I) are being compared with infants receiving 5 or more days of Mg (Group II). Initially there was little difference in the Apgar scores and the severity of illness in the two groups. However, Group I required significantly longer hospitalization and had more periods of recurrent apnea and bradycardia following discharge (2P less than 0.001). A time-series analysis of the relation of Mg therapy to apnea and bradycardia is being conducted. Congenital magnesium (Mg) deficiency appears to be an entirely different syndrome from dietary Mg deficiency. Pups borne and suckled by dams fed Mg doses ranging from 5 to 150 mg of magnesium/100 g diet during pregnancy and lactation were studied on days 10, 13 and 16 of neonatal life. A small but significant change was found in bone Mg. (However, pups that were normal at birth and then subjected to dietary Mg deficiency had only 1/3 of the control level of Mg in bone). Bone is an unsatisfactory tissue for diagnosing congenital Mg deficiency but is helpful for diagnosing dietary Mg deficiency. Weanling and young adult rats fed marginal dietary Mg (10 mg/100) or control (40 mg/100 g) were studied to learn the effect of repeated injections of furo- semide (F) on Mg and calcium (Ca) metabolism. The rats fed the lower dietary Mg showed a significant increase in urinary Mg and Ca after F injections, and they developed signs of Mg deficiency, including reduced weight gain, severe skin ulcerations, and increased Ca in heart and kidney. Prolonged F therapy may result in Mg deficiency if Mg losses are not adequately repleted.